This invention relates to the field of crystal growth and annealing, specifically a method and apparatus for crystal growth and annealing with minimized residual stress and suitable for production of calcium fluoride (CaF.sub.2) crystals.
Crystals are used in a wide variety of applications, including as lenses in digital broadcast cameras and as optical elements in lithography such as in semiconductor processing. Semiconductor lithography at 193 nm wavelengths commonly used fused silica optical elements. Unfortunately, fused silica is damaged by high fluence at 193 nm. The next generation of semiconductor lithography is expected to use 157 nm wavelength illumination. Another material will be required since fused silica is quite opaque to 157 nm wavelength illumination.
CaF.sub.2 is one of several candidates for optical elements in 193 nm and 157 nm lithography. Current crystal growth and annealing processes lead to high residual stress in large CaF.sub.2 crystals, however, limiting the applicability of CaF.sub.2 crystals. High residual stresses in a crystal can cause the crystal to exhibit a spatially varying index of refraction. This can lead to wavefront errors, image degradation, and birefringence, all detrimental to the effectiveness of an optical system using CaF.sub.2.
Contemporary crystal growth and annealing is illustrated by FIG. 1(a,b). A powder P is placed in a crucible C. During the growth phase, the powder P is heated to a liquid phase (roughly 1500.degree. C. for CaF.sub.2, for example). The crucible C is slowly lowered from the heated region R1, with the crystal X growing in the region R2 where the liquid can cool below a critical temperature. The difference between the liquid temperature T1 and crystal temperature T2 leads to a temperature gradient across the crystal/liquid combination.
Once the crystal growth phase is complete, the crystal X is annealed. FIG. 1b shows the arrangement in a conventional annealing process. The crystal X is placed back in the heated region R1, but the temperature is less than that required to liquefy the crystal X. The crystal loses heat through its top, bottom, and sides. The temperature of the crystal is slowly reduced until it reaches a certain value, typically room temperature (annealing a CaF.sub.2 crystal conventionally takes approximately 30 days to bring the temperature from 1000.degree. C. to 50.degree. C., at cooling rates of less than 1.degree. C. per hour). The temperature of the crystal is slowly reduced, conventionally still with a vertical temperature gradient as represented by differences between T1 and T2. After the crystal is completely cooled, typically the top and bottom are cut off to produce a blank. The blank can then be ground and polished to produce an optical element such as a lens, tube, or plate.
Current CaF.sub.2 crystal production methods reliably produce CaF.sub.2 crystals of limited size, because the CaF.sub.2 crystals produced exhibit unacceptably high birefringence at sizes over about 6 inch diameter. The limited size crystals limit the numerical aperture available with resulting optical elements, limiting the optical elements' utility for high density lithography.
Accordingly, there is a need for a method and apparatus for producing crystals that minimizes birefringence even at large crystal sizes, and is suitable for production of CaF.sub.2 crystals.